U.S. patent application number 16/167760 was filed with the patent office on 2019-04-25 for control system and method for operating a plurality of wind turbines.
The applicant listed for this patent is SENVION GmbH. Invention is credited to Jens Geisler, Jesse Ben Benjamin Schmuck.
Application Number | 20190120209 16/167760 |
Document ID | / |
Family ID | 63965293 |
Filed Date | 2019-04-25 |
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United States Patent
Application |
20190120209 |
Kind Code |
A1 |
Geisler; Jens ; et
al. |
April 25, 2019 |
Control System and Method for Operating a Plurality of Wind
Turbines
Abstract
A method for operating a first wind turbine and a second wind
turbine, the second wind turbine being located in the wake of the
first wind turbine. A prediction model is fed with a current wind
value of the first wind turbine, in order to predict a future time
point at which the area swept by the rotor of the second wind
turbine becomes partially overlapped by the wake of the first wind
turbine. The second wind turbine reacts to the prediction in that a
control signal is generated in order to alter the pitch angle of a
rotor blade of the second wind turbine relative to the pitch angle
of another rotor blade of the second wind turbine. The invention
additionally relates to a control system suitable for executing the
method. Implementation of the disclosed method by a control system
can reduce the loading of the second wind turbine.
Inventors: |
Geisler; Jens; (Rendsburg,
DE) ; Schmuck; Jesse Ben Benjamin; (Flensburg,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SENVION GmbH |
Hamburg |
|
DE |
|
|
Family ID: |
63965293 |
Appl. No.: |
16/167760 |
Filed: |
October 23, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F05B 2260/84 20130101;
F05B 2260/8211 20130101; F05B 2260/821 20130101; F05B 2260/80
20130101; F05B 2270/404 20130101; F05B 2270/111 20130101; F05B
2270/321 20130101; F03D 7/048 20130101; F03D 7/045 20130101; F03D
7/0224 20130101; F05B 2270/20 20130101; G05B 13/048 20130101; F03D
1/00 20130101; F05B 2270/32 20130101; F05B 2220/30 20130101 |
International
Class: |
F03D 7/04 20060101
F03D007/04; F03D 1/00 20060101 F03D001/00; G05B 13/04 20060101
G05B013/04 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 2017 |
DE |
10 2017 009 838.7 |
Claims
1. A method for operating a first wind turbine (14) and a second
wind turbine (15), the second wind turbine (15) being located
downwind of the first wind turbine (14), said method comprising:
providing a current wind value of the first wind turbine (14) to a
prediction model (28), said current wind value comprising a current
wind speed value or a current wind direction value; in said
prediction model (28), using the current wind value of the first
wind turbine (14) to predict a future time point (25) at which an
area swept by a rotor of the second wind turbine (15) overlaps a
wake (30) of the first wind turbine (14); in response to said
prediction, generating a control signal to alter a pitch angle of a
rotor blade of the second wind turbine (15) relative to the pitch
angle of another rotor blade of the second wind turbine (15).
2. The method of claim 1, wherein the control signal is generated
before the future time point (25).
3. The method of claim 1, wherein the prediction model (28)
comprises an assumption that the wake (30) widens in a radial
direction as a distance from the first wind turbine (14)
increases.
4. The method of claim 1, wherein the prediction model (28)
comprises an assumption that a wind speed in the wake (30) is
reduced in comparison with a wind field that is not disturbed by
the first wind turbine (14).
5. The method of claim 1, wherein the prediction model (28)
comprises an assumption that the wind speed in the wake (30) has a
radial distribution according to which a wind speed in the center
of the wake (30) is less than in the periphery of the wake
(30).
6. The method of claim 1, wherein the prediction model (28)
comprises an assumption that the turbulence intensity in the wake
(30) is increased in comparison with a wind field that is not
disturbed by the first wind turbine (14).
7. The method of claim 1, comprising determining an estimated wind
value on the basis of an air mass-flow acting upon the entire rotor
of the first wind turbine (14); and using the estimated wind value
as the current wind value of the first wind turbine (14).
8. The method of claim 1, wherein the current wind value of the
first wind turbine (14) and a current wind value the second wind
turbine (15) are used to predict a propagation direction of the
wake (30).
9. The method of claim 1, wherein the prediction model maps a wind
prediction into a geographical coordinate system.
10. The method of claim 9, wherein a location of the first wind
turbine (14) and the location of the second wind turbine (15) are
defined by coordinates within the geographical coordinate
system.
11. The method of claim 1, wherein the current wind value comprises
a value for turbulence intensity derived from a standard deviation
of a current output power of the first wind turbine (14).
12. The method of claim 1, wherein the prediction model (28)
determines a time span within which a change in wind direction
propagates from the first wind turbine (14) to the second wind
turbine (15) as the sum of a time lag, which is derived from the
current wind speed value and a wake-related additional time.
13. The method of claim 1, comprising: equipping the second wind
turbine (15) with load sensors to determine an actual wind load
acting on the second wind turbine (15).
14. The method of claim 1, wherein said method is active only when
a present wind speed [at said first turbine (14)] is above the
nominal speed or the second wind turbine (15) is presently being
operated at nominal output.
15. The method of claim 1, comprising: determining a sequence of
current wind values at different time points for the first wind
turbine (14) or the second wind turbine (15); and storing the
sequence of current wind values for the first wind turbine (14) or
the second wind turbine (15).
16. A control system for wind turbines, said control system
comprising: an acquisition means for recording a current wind value
of a first wind turbine (14), a prediction model (28) that
processes the current wind value to predict a future time point
(25) at which a second wind turbine (15), located in a wake (30) of
the first wind turbine (14), is overlapped by the wake (30) of the
first wind turbine (14), wherein, in response to said prediction,
the control system generates a control signal to alter a pitch
angle of a rotor blade of the second wind turbine (15) relative to
a pitch angle of another rotor blade of the second wind turbine
(15).
Description
BACKGROUND
[0001] The invention relates to a method for operating a first wind
turbine and a second wind turbine, the second wind turbine being
located in the wake of the first wind turbine. The invention
additionally relates to a control system for wind turbines.
[0002] If two wind turbines are operated adjacently to each other,
there arises the problem that inconsistent wind conditions, by
which the second wind turbine may be subjected to greater loading,
may prevail in the wake of the first wind turbine, EP 2 063 108 A2,
EP 1 790 851 A2, WO 2015/039 665 A1, WO 2011/160 634 A1.
SUMMARY OF THE INVENTION
[0003] The invention is based on the object of presenting a control
system and a method for operating a plurality of wind turbines,
such that the loading of the second wind turbine located in the
wake of a first wind turbine is reduced. Proceeding from the known
prior art, the object is achieved with the features of the
independent claims. Advantageous embodiments are specified in the
dependent claims.
[0004] In the case of the method according to the invention, a
prediction model is fed with a current wind value of the first wind
turbine, in order to predict a future time point at which the area
swept by the rotor of the second wind turbine becomes partially
overlapped by the wake of the first wind turbine. The second wind
turbine reacts to the prediction in that a control signal is
generated in order to alter the pitch angle of a rotor blade of the
second wind turbine relative to the pitch angle of another rotor
blade of the second wind turbine. In one embodiment, the control
signal is generated before the future time point.
[0005] If the wake of a first wind turbine impinges only partially
on the rotor of a second wind turbine, the rotor of the second wind
turbine is loaded asymmetrically. The invention is based on the
concept of counteracting the asymmetrical loading of the rotor, in
that one of the rotor blades is adjusted relative to the other
rotor blades, the adjustment being effected preventatively, i.e. at
a time point before the increased loading acts upon the rotor
(pre-control).
[0006] Embodiments of the disclosed prediction model may
incorporate assumptions applied to information provided to the
prediction model. According to aspects of the invention,
assumptions may be implemented as computational rules based upon
physical laws governing the behaviour of fluids such as the air
mass-flow acting upon a wind turbine. For example, the prediction
model may be based on the assumption that the wake extends downwind
from the rotor of the first wind turbine along an extension of the
rotor axis of the first wind turbine. The area swept by the rotor
of the first wind turbine may be assumed as a starting point for
the cross section of the wake. The prediction model may be based on
the assumption that, starting from this starting point, the wake
propagates with a cylindrical cross section along the direction of
the rotor axis. The prediction model may be based on the assumption
that the wake widens in the radial direction as the distance from
the rotor of the first wind turbine increases. The prediction model
may be based on the assumption that the wind speed in the wake is
reduced in comparison with a wind field that is not disturbed by
the first wind turbine. The prediction model may be based on the
assumption that the wind speed in the wake has a radial
distribution according to which the wind speed in the center of the
wake is less than in the periphery of the wake. The prediction
model may be based on the assumption that the turbulence intensity
in the wake is increased in comparison with a wind field that is
not disturbed by the first wind turbine.
[0007] The prediction model may be based on the assumption that,
starting from the first wind turbine, the wake propagates at a
speed that corresponds to the wind speed. The prediction model may
be fed with a wind speed value from which the propagation speed of
the wake is determined.
[0008] The prediction model may be based on the assumption that the
wake propagates in the wind direction. The prediction model may be
fed with a wind direction value from which the propagation
direction of the wake is determined.
[0009] The current wind speed value and/or the current wind
direction value may be measured values. A measured value may be
obtained by means of an anemometer. The anemometer may be arranged,
for example, on the nacelle of a wind turbine.
[0010] It is also possible for the current wind speed value and/or
the current wind direction value to be an estimated value. In many
cases, values estimated on the basis of established parameters have
greater reliability, compared with measured values. For greater
reliability it is advantageous if the estimated values are derived
from the air-mass flow acting upon the entire rotor. A
substantially greater measurement area is thus available, in
comparison with an anemometer arranged on the wind turbine. In
particular, the estimated values may be derived from operating
parameters of the wind turbine, which are selected such that there
is a dependence between the operating parameters and the air-mass
flow acting upon the rotor. Thus, if the air-mass flow acting upon
the rotor changes, there is a resultant change, dependent thereon,
in the operating parameters. Possible operating parameters of a
wind turbine that may be taken into account in the determination of
an estimated wind value are, for example, the rotational speed of
the rotor, the pitch angle of one or more rotor blades, the torque
of the generator, an inclination of the tower and/or the
deformation of a rotor blade. In particular, a change in one of
these quantities may be included in the determination of an
estimated wind value. A plurality of these operating parameters, in
particular at least three of these operating parameters, may be
taken into account in the determination of the estimated values.
Furthermore, an estimated air density, or air density determined on
the basis of measurement data, may advantageously be taken into
account.
[0011] Wind turbines typically include sensors and measurements of
the condition and operating parameters of a wind turbine on a
periodic or continuous basis, so data corresponding to the
operating parameters of the wind turbine are available for any
given point in time. This data is communicated to a computer and
processed to determine a current estimated wind value for a wind
turbine at a time point. The current estimated wind value may
include a current estimated wind speed value and/or a current
estimated wind direction value at a time point. According to
aspects of the invention, the current estimated wind value may be
used as a current wind value in the prediction model.
[0012] The current wind speed value and/or the current wind
direction value may be obtained in the case of the first wind
turbine. The prediction model can thus be based on the assumption
that the wake propagates with the wind direction acting upon the
first wind turbine.
[0013] The current wind speed value and/or the current wind
direction value may be obtained in the case of the second wind
turbine. The prediction model can thus be based on the assumption
that the wake of the first wind turbine propagates according to the
wind direction in the case of the second wind turbine.
[0014] The prediction model may be designed such that, in the
prediction of the propagation direction of the wake, both a current
wind direction value of the first wind turbine and a current wind
direction value of the second wind turbine are taken into account.
For example, a propagation direction of the wake that is between
the two wind directions may be assumed if the current wind
direction value of the first wind turbine and the current wind
direction value of the second wind turbine differ from each
other.
[0015] The prediction model may be designed such that, in the
prediction of the propagation speed of the wake, both wind speed
values of the first wind turbine and wind speed values of the
second wind turbine are taken into account. For example, a
propagation speed of the wake that is between the two wind speeds
may be assumed if the wind direction values of the first wind
turbine and of the second wind turbine differ from each other.
[0016] Once the prediction model has estimated a propagation speed
and a propagation direction for the wake, it can then be predicted,
from the relative positions of the first wind turbine and the
second wind turbine in relation to each other, at which future time
point the second wind turbine is subjected to a particular effect
of the wake of the first wind turbine.
[0017] The geographical positions of the first wind turbine and of
the second wind turbine may be represented in a coordinate system.
If the predicted wake is mapped into the coordinate system, the
degree of overlap between the wake and the rotor of the second wind
turbine can be determined. In particular, the axial distance
between the first wind turbine and the second wind turbine,
measured along an axis of the wake, can be determined. In addition,
a radial distance between an axis of the wake and the axis of the
second wind turbine can be determined. The degree of overlap
between the wake and the area swept by the rotor of the second wind
turbine is obtained from the radial distance.
[0018] The wake model may comprise a module for predicting the
turbulence intensity. For this purpose, a current value of the
turbulence intensity in the case of the first wind turbine can be
obtained. The propagation of the turbulence intensity may be
estimated by means of the prediction model, and a prediction for
the turbulence intensity at a future time point in the case of the
second wind turbine derived therefrom. In this case, it may be
taken into account in the prediction model that the turbulence
intensity in the wake is generally higher than in an undisturbed
wind field. The current value of the turbulence intensity in the
case of the first wind turbine may be derived, for example, from a
standard deviation of the output power of the first wind
turbine.
[0019] In a simple variant, in the case of a change in the current
wind direction in the region of the first wind turbine, the
prediction model may proceed such that the change in the wind
direction is transferred directly to the wake. This means that the
predicted wake changes its direction simultaneously with the wind
direction. The speed at which the wake moves in the new direction
may be derived from the current wind speed. The time lag after
which the wake has propagated in the new direction, as far as the
second wind turbine, can be predicted on the basis of the axial
distance between the first and the second wind turbine. The sum of
the current time point and the time lag gives the future time point
to which the prediction applies.
[0020] Analyses have shown that, in practice, changes in the wind
direction only continue into the wake with a certain time lag. This
can be taken into account in the prediction model, in that the time
span within which a change in wind direction propagates as far as
the second wind turbine is determined as the sum of a time lag,
which is derived from the current wind speed value, and a
wake-related additional time, which results from the fact that the
wake does not directly follow a change in the wind direction.
[0021] Precise determination of the additional time is not entirely
simple. On the other hand, the exact prediction of the future time
point and of the degree of overlap between the wake and the rotor
of the second wind turbine is crucial for the success of the
method. If there is a pre-control in the second wind turbine in the
expectation of an imminent wake loading, and the wake then does not
actually occur, the resultant loading of the wind turbine may be
greater than if the pre-control had been omitted. In order to avoid
such negative effects of the method according to the invention, the
second wind turbine may be equipped with load sensors, by which the
actual loading of the wind turbine is determined. The loading
sensors may comprise, for example, strain gauges in the rotor
blades, by which an elastic deformation of the rotor blade can be
measured.
[0022] In this way, the prediction model can be improved in an
automatic learning process. If the control signal derived from the
prediction model does not result in the expected reduction in the
loading, a corresponding response message can be sent to the
prediction model, such that the latter adapts its prediction. In
other words, the prediction model can be modified in dependence on
a measured actual loading of the second wind turbine. The
measurement of the actual loading may be effected, for example, at
the future time point. The modification of the prediction model may
relate, in particular, to the time span within which a change in
the wind direction has propagated from the first wind turbine as
far as the second wind turbine.
[0023] If, for a future time point, the prediction model predicts a
state in which the wake of the first wind turbine partially
overlaps the rotor of the second wind turbine, the pitch angle of a
rotor blade of the second wind turbine is altered relative to the
pitch angle of another rotor blade of the second wind turbine. The
alteration may relate to the sector of the rotor revolution in
which the covering by the wake of the first wind turbine occurs. In
other sectors of the rotor revolution, the rotor blade can return
to its previous position. This sequence may be repeated cyclically,
such that, upon reaching the same angular position, the rotor blade
again adjusts its pitch angle. A corresponding control
specification may also be effected for the other rotor blades of
the rotor, such that the asymmetrical loading is counteracted by a
cyclic adjustment of all rotor blades.
[0024] Each wind turbine has a rated electrical energy output,
which corresponds to the maximum electrical power the wind turbine
can deliver in continuous operation. This maximum electrical power
can be referred to as the "nominal output" of the wind turbine. The
minimum wind speed necessary for the turbine to generate its
nominal electrical output can be referred to as the "nominal wind
speed" for the turbine. A cyclic adjustment of the rotor-blade
pitch angle is normally associated with a reduction in power drawn
from the wind. In order to keep the yield losses low, the second
wind turbine may be configured such that the pre-control on the
basis of the prediction model is performed only when the present
wind speed is above the nominal wind speed, and/or the second wind
turbine is presently being operated at nominal output.
[0025] It may be advantageous for the method if a sequence of wind
direction values and/or wind speed values, which have been
determined at differing time points, is stored for the first wind
turbine and/or the second wind turbine. Each of the wind direction
values/wind speed values may be provided with a time stamp, such
that the time point at which the value was recorded is verifiable.
The time period over which the sequence of stored values extends
may be fixed such that it includes the period required by a wind
field, in the case of the lowest wind speed to be taken into
consideration, to move from the first wind turbine as far as the
second wind turbine. A ring buffer, in which the data are stored
over the required time span, and in which older data are
automatically replaced by newly received data, may be used for
storing the data.
[0026] These wind speed values/wind direction values may be stored
together with the associated time stamp. In addition, information
about the current operating status of the wind turbine concerned
may be stored together with the time stamp.
[0027] The computing steps necessary for the application of the
prediction model may be performed on a central computer of the wind
park, to which the first wind turbine and the second wind turbine
belong. However, it is also possible to do this in a decentralized
manner, by means of a computer of a wind turbine.
[0028] The method according to the invention may be executed with
more than two wind turbines. In particular, the method may be
executed with at least 10, preferably at least 20, more preferably
at least 50 wind turbines. In the case of the arrangement of wind
turbines according to the invention, the wind turbines are
spatially arranged in relation to each other such that, depending
on the wind direction, each wind turbine can come into the wake of
at least one other wind turbine of the arrangement. The arrangement
of wind turbines may be configured as a wind park, having a central
controller, which issues specifications for the operation of
individual wind turbines.
[0029] If the method is executed with more than two wind turbines,
the method according to the invention may comprise the step,
dependent on the wind direction, of performing a sorting of the
wind turbines, according to which the wind turbines are sorted into
pairs of first and second wind turbines, such that in each pair, in
the case of the present wind direction, the second wind turbine is
located in the wake of the first wind turbine. It is possible in
this case for an individual wind turbine in a first pair to assume
the role of the first wind turbine, and at the same time, in a
second pair, to assume the role of the second wind turbine.
[0030] The invention additionally relates to a control system,
having an acquisition means for recording a current wind value of a
first wind turbine, and having a prediction model, which is
designed, by processing the current wind value, to predict a future
time point at which a second wind turbine, located in the wake of
the first wind turbine, is partially overlapped by the wake of the
first wind turbine, and in which, in reaction to the prediction,
the control system generates a control signal, in order to alter
the pitch angle of a rotor blade of the second wind turbine
relative to the pitch angle of another rotor blade of the second
wind turbine. The invention additionally relates to an arrangement
of wind turbines, which is equipped with such a control system.
[0031] In a preferred embodiment, the control system has a
plurality of control computing facilities. The wind prediction in
this case is preferably performed on a central computing facility
for the entire wind park. The respective wind prediction of an
individual wind turbine is transmitted from the central computing
facility to the control units of a plurality of individual wind
turbines. The respective control units of the individual wind
turbines then generate control signals for the individual wind
turbines from the transmitted wind prediction.
[0032] The control system may be enhanced with further features,
which are described in connection with the method according to the
invention. The method may be enhanced with further features, which
are described in connection with the control system according to
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The invention is described exemplarily in the following with
reference to the appended drawings, on the basis of advantageous
embodiments. There are shown:
[0034] FIG. 1: an arrangement of wind turbines according to the
invention;
[0035] FIG. 2: a schematic representation of a prediction model
according to the invention;
[0036] FIG. 3: a partial overlap between the wake of a first wind
turbine and the rotor of a second wind turbine;
[0037] FIG. 4: an aspect of the prediction model according to the
invention;
[0038] FIG. 5: an arrangement of wind turbines according to the
invention; and
[0039] FIG. 6: a flow chart showing an example of steps in applying
a prediction model according to aspects of the invention.
DETAILED DESCRIPTION
[0040] An arrangement of wind turbines shown in FIG. 1 comprises a
first wind turbine 14, a second wind turbine 15 and a third wind
turbine 16. Each wind turbine 14, 15, 16 comprises a rotor, which
is put into rotation by the wind and which drives a generator for
the purpose of generating electrical energy. Each of the wind
turbines 14, 15, 16 additionally comprises a wind estimator 17,
which in FIG. 1 is represented only on the example of the wind
turbine 15.
[0041] The wind estimators 17 are designed to determine, for each
of the wind turbines 14, 15, 16, an estimated value for the wind
speed and possibly also for the wind direction. Each estimated
value is derived from one or more operating parameters, which
change in dependence on the air-mass flow acting upon the entire
rotor. These operating parameters may comprise, for example, the
rotational speed of the rotor, the pitch angle of the rotor blades
and the torque applied by the rotor. Since the rotor sweeps a large
area, reliable values for the wind speed, and optionally also those
for the wind direction, can be estimated by means of the wind
estimator 17. The latter requires additional measurement values,
preferably, for example, a measurement of the individual blade
motion or deformation, or a measurement of the tower motion or
deformation.
[0042] According to aspects of the invention, a control system
implementing the disclosed prediction model 28 includes acquisition
means for recording a current wind value of a first wind turbine
(14). Acquisition means includes sensors and measurements of the
condition and operating parameters of a wind turbine, a system for
communicating this data to a computer, and calculations performed
on the sensor readings and measured values to determine a current
estimated wind value for a wind turbine at a time point. A wind
turbine 15 may be equipped with load sensors 11 to determine an
actual wind load acting on the wind turbine 15. A current estimated
wind value may be used in the prediction model as the current wind
value according to aspects of the invention.
[0043] In the case of the wind direction 18 prevailing in FIG. 1,
the second wind turbine 15 is located downwind from the first wind
turbine 14. The distance between the second wind turbine 15 and the
first wind turbine 14, as viewed in the wind direction, is such
that the second wind turbine 15 is located in the wake 30 of the
first wind turbine 14. Thus, in the case of the wind direction 18,
the wind conditions experienced by the second wind turbine 15 are
influenced by the operation of the first wind turbine 14.
[0044] Although the third wind turbine 16 is also located upwind of
the second wind turbine 15, the third wind turbine 16 is offset so
far to the side that there is no intersection between the wake 30
of the third wind turbine 16 and the rotor of the second wind
turbine 15. At the start of the method according to the invention,
it is ascertained, taking into account the current wind direction,
which wind turbines are located in the wake of which other wind
turbines. In the exemplary embodiment according to FIG. 1, only the
second wind turbine 15, located in the wake of the first wind
turbine 14, is affected.
[0045] Estimated wind values of all three wind turbines 14, 15, 16
are determined, at the current time point 20, on the time axis 19
in FIG. 1. The estimated wind values are passed to a prediction
computer 21, in which there is a stored prediction model 28. The
estimated wind values are fed into the prediction model 28, which
processes the estimated wind values in order to predict how the
wake 30 of the first wind turbine 14 will spread.
[0046] A propagation direction and a propagation speed of the wake
are determined by means of the prediction model 28. In the example
according to FIG. 1, the propagation direction is derived from the
current wind direction 18, and the propagation speed from the wind
speed. In FIG. 1 this is represented schematically by a front line
22, which is aligned parallel to the rotor of the first wind
turbine 14 and which moves downwind. A mean value of the wind
direction estimated in the case of the wind turbines 14, 16 is
assumed as a current wind direction. The flow received by both wind
turbines 14, 16 is unobstructed, such that the wind direction is
not falsified by the wake of a wind turbine located farther to the
front. In other embodiments of the invention, an estimated value
obtained in the case of the second wind turbine 15 may also be
taken into account in the determination of the current wind
direction. A corresponding procedure may be applied in determining
the current wind speed.
[0047] A control unit 24 of the second wind turbine 15 may request,
from the prediction computer 21, how the wake 30 of the first wind
turbine 14 is likely to affect the second wind turbine 15 at the
future time point 25. In the exemplary embodiment according to FIG.
1, at the future time point 25 there is a partial overlap between
the wake 30 of the first wind turbine 14 and the rotor of the
second wind turbine 15.
[0048] As a reaction to this prediction, shortly before the time
point 25 the control unit 24 generates a control signal, according
to which the pitch angle of the rotor blades of the second wind
turbine 15 is adapted, before the time point 25, in dependence on
the angular position of the respective rotor blade (pre-control).
In particular, in the angular range of the rotor revolution in
which there is an intersection with the wake 30 of the first wind
turbine 14, each rotor blade may have a pitch angle that differs
from that in other angular ranges of the rotor revolution. The
control signal of the control unit 24 can thus effect a cyclic
alteration of the pitch angle, and the control signals can be
repeated after their complete rotor revolution.
[0049] A current value of the turbulence intensity may additionally
be determined at the current time point 20. The turbulence
intensity may be determined, for example, from the standard
deviation of the output power delivered by the first wind turbine
14. The prediction computer 21 may be designed such that it also
determines a propagation of the turbulence intensity, and makes a
corresponding prediction for the future time point 25. The control
specification of the control unit 24 for the second wind turbine 15
may also depend on the turbulence intensity predicted for the time
point 25.
[0050] Illustrated in FIG. 2 are some assumptions on which the
prediction model 28 for the wake is based. If constant wind
conditions are assumed, the wake 30 propagates rearwards, behind
the rotor blade of the first wind turbine 14, concentrically in
relation to the rotor axis. In the region of the first wind turbine
14, the cross section of the wake 30 corresponds to the area swept
by the rotor. As the distance from the first wind turbine 14
increases, the wake 30 widens slightly in the radial direction.
[0051] In the region of the wake 30 the wind speed is less than it
would be in a wind field not disturbed by the first wind turbine
14. The wind speed deficit is greatest in the region directly
adjoining the rotor (with dark coloring). As the distance from the
first wind turbine 14 increases in the downwind direction, and as
the distance from the central axis of the wake 30, represented by a
broken line, increases, the wind speed deficit becomes less.
Accordingly, in FIG. 2, the second wind turbine 15 is affected only
by a slight wind speed deficit, in the edge region of the wake
30.
[0052] In FIG. 3, the area swept by the rotor of the second wind
turbine 15 is identified by a circle 22. According to the
prediction model 28, the wake 30 likewise has a circular cross
section in the plane of the rotor of the second wind turbine 15.
Between the wake 30 and the area 22 swept by the rotor there is a
partial overlap, in the region 23.
[0053] FIG. 4 shows an exemplary possibility of how changes in the
wind direction can be mapped in the prediction model 28.
Represented is a period between a current time point t and a past
time point t-3.DELTA.t. At the past time point t-3.DELTA.t the wind
came from 270.degree., such that the second wind turbine 15 was
located entirely in the wake 30 of the first wind turbine 14.
Between t-3.DELTA.t and t-2.DELTA.t the wind direction turned by
10.degree., to 260.degree.. Owing to the wake 30, there is a
resultant delay until this change in the wind direction reaches the
second wind turbine 15. Between t-2.DELTA.t and t-.DELTA.t the wind
turns by a further 10.degree., to 250.degree., and maintains this
new wind direction up to the current time point t. The graphic
indicates the respective delay until the changed wind direction has
propagated into the wake 30.
[0054] In the case of the wind park represented schematically in
FIG. 5, the wind turbines located upwind, as first wind turbines
14, are represented separately from the second wind turbines 15
located downwind. The basic pair-wise allocation between first wind
turbines 14 and second wind turbines 15 is dependent on the wind
direction, and must be adapted upon each change in the wind
direction. All wind turbines 14, 15 are equipped with wind
estimators 17, which send estimated wind values to a central buffer
33. In the central buffer 33, the estimated wind values are stored
together with the geographical coordinates of the respective wind
turbine and with an associated time stamp. In addition to this, in
each case information may be stored about the operating state of
the wind turbine concerned at the respective time point.
[0055] The data are stored in the buffer 33 for as long as is
required by the slowest wind field to be considered to move over
the greatest extent of the wind park. The slowest wind field to be
considered may have a speed, for example, corresponding to the
cut-in wind speed of the wind turbines. The buffer 33 is realized
as a ring buffer, such that the oldest data of this period are in
each case replaced by newer data.
[0056] Each wind turbine 14, 15 is equipped with a decentralized
prediction computer 35, which can access the estimated wind values
in the buffer 33. For each second wind turbine 15 located downwind,
a geographical sector, in which a first wind turbine 14 could
generate a disturbing wake, is obtained from the current wind
direction. By requesting the geographical coordinates of the first
wind turbines 14 that are stored in the buffer 33, the prediction
computer can identify individual wind turbines 14 that potentially
could cause a disturbing wake. As a result of the prediction
computer 35 accessing the current estimated wind values of the
respective first wind turbine in the buffer 33, the prediction
computer can predict whether a disturbing wake 30 is soon to be
anticipated for its own wind turbine 15. If this is the case,
shortly before the arrival of the disturbing wake the wind turbine
15 can generate a control signal, according to which the rotor
blades are pitched cyclically.
[0057] With reference to FIG. 6, the method step of applying the
prediction model is explained in more detail. The method starts in
step 100. In step 110 a current value of the wind speed and a
current value of the wind direction at the first wind turbine 14
are obtained.
[0058] In step 120 a wake 30 of the first wind turbine 14 is
determined so that a central axis 36 of the wake is coaxial to the
rotor axis of the first wind turbine 14 and so that the wake 30 is
circular in cross-section. The wake 30 may have a length 39 that
corresponds to four times the rotor diameter of the first wind
turbine 14.
[0059] In one embodiment the wake 30 has a constant diameter over
its length 39, wherein the diameter corresponds to the diameter of
the rotor of the first wind turbine 14. In another embodiment the
diameter of the wake 30 increases with increasing distance from the
first wind turbine 14.
[0060] In step 130 an axial distance 38 between the first wind
turbine 14 and the second wind turbine 15 is determined. In step
140 a radial distance 37 between the central axis 36 of the wake 30
and the rotor axis of the second wind turbine 15 is determined.
[0061] In step 150 it is determined whether there is an overlap
between the wake 30 of the first wind turbine 14 and the rotor of
the second wind turbine 15. This is done by comparing the length 39
of the wake 30 with the axial distance 38 between the first wind
turbine 14 and the second wind turbine 15 and by comparing the
radial distance 37 with the radius of the rotor of the second wind
turbine 15 and with the radius of the wake 30. If the axial
distance 38 is smaller than the length 39 of the wake 30 and if the
radial distance 37 is smaller than the sum of the radius of the
rotor of the second wind turbine 15 and the radius of the wake 30
at the same axial position, there is an overlap between the wake 30
and the rotor of the second wind turbine 15. If one of the two
conditions is not met there is no overlap between the wake 30 and
the rotor of the second wind turbine 15. In this case the method
starts over with step 100.
[0062] If there is an overlap the method proceeds to step 160. In
step 160 a future time point 25 is determined when the second wind
turbine 15 is hit by the currently determined wake 30 of the first
wind turbine. This is done by dividing the axial distance 38
between the first wind turbine 14 and the second wind turbine 15 by
the propagation speed of the wake, which propagation speed in one
embodiment corresponds to the wind speed obtained in step 110.
[0063] In step 170 a control signal is generated at time point 25
or before time point 25 for altering a pitch angle of a rotor blade
of the second wind turbine 15 relative to the pitch angle of
another rotor blade of the second wind turbine 15. The method ends
in step 180.
[0064] Steps 120, 130, 140, 150, 160 are an exemplary mode of
predicting a future time point 25 at which there is an overlap
between a wake of the first wind turbine 14 and an area swept by a
rotor of the second wind turbine 15. Other embodiments of
determining the wake 30 and of determining the future time point 25
are described in this specification.
* * * * *